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Jet engine turbines - how they work?

Hi,
I know how jet engines work except for one aspect and for the purpose of this question I'll use
http://en.wikipedia.org/wiki/Turbojet
for the discussion so we're all picturing the same thing and because the illustration shows the engine quite simply.

By looking at the picture, I understand that turbojet can't start itself, it needs 'spinning up' before it is ignited at which point it should sustain itself spinning and then provide thrust.
I understand that because the turbine is spinning, the compressors at the front compress air into the combustion chamber, where fuel is injected and then ignited causing expansion, driving the rear turbines to drive the front compressor, and also providing thrust.

What I don't understand is why the expansion in the combustion chamber isn't equally providing thrust in both forward and backward directions, and why it isn't acting on the compressors and causing them to be turbines just like at the rear of the engine.  What directs the expansion towards the back of the engine? and what stops half (or at least a large fraction) of it pushing back towards the front of the engine?
I know rockets have an expansion but it's usually a tube with one end closed so all of the expansion is forced out of one end, but both ends are open on a jet engine?

The question arose when I was looking at tin can turbines
http://www.youtube.com/watch?v=2c1i_UIEsSM&feature=related
and I was thinking how I would build a small one myself but I couldn't visualise why the combustion isn't pressing against both the front and rear propeller cancelling any rotation out, and providing equal thrust in both directions?

Thanks for your time!

blkhawk5 years ago
Maybe this article in the How Stuff Works site about jet engines might help.
arichardson (author)  blkhawk5 years ago
Hi blkhawk,
Thanks for your link, it showed a better diagram of the combustion chamber which helped a bit, Thanks!
The burning happens inside combusition chambers attached to the engine frame, there is no action to the front compressor blades.
arichardson (author)  steveastrouk5 years ago
Hi steve, what I was struggling to get my head around was how air gets into the combustion chamber from the front of the engine but then can't get back out the same way it came in (there is no system of valves) but I'm thinking it's all down to the design of the combustion chamber where most of the expansion is directed backwards.
But surely the high pressure air before the combustion isn't as high pressure as during combustion, so the pressure difference would cause no more air to be drawn into the engine until the combustion calms down a bit. The engine would then only run with very high frequency pulses?